The switched reluctance machine is a type of brushless electrical machine. It comprises a rotor, defining rotor poles, a stator, defining stator poles, and a set of windings arranged in relation to the stator poles to define one or more independently energisable phases. In a reluctance machine, energisation of one or more phase windings sets up magnetic flux in a circuit which includes the associated stator poles, urging the rotor into a position where the reluctance of the circuit is a minimum (and the inductance of the associated phase winding is a maximum). In motoring operation, timing the sequential energisation of the windings according to rotor position induces rotor movement. A general treatment of electrical drives which incorporate switched reluctance machines can be found in various textbooks, e.g. “Electronic Control of Switched Reluctance Machines” by T J E Miller, Newnes, 2001 which is incorporated herein by reference. More detail is provided in the paper ‘The Characteristics, Design and Applications of Switched Reluctance Motors and Drives’ by Stephenson and Blake, presented at the PCIM '93 Conference and Exhibition at Nurnberg, Germany, Jun. 21-24, 1993 which is incorporated herein by reference. As is well known in the art, these machines can be operated as motors or generators by altering the timing of the application of the energisation to the phase windings.
Unlike conventional induction and synchronous ‘electromagnetic’ machines, e.g., so-called brushless DC machines, in which the current is in stator coils and the field is produced by permanent magnets on the rotor, switched reluctance machines are purely ‘magnetic’ machines. The torque is produced solely by the magnetic field as the reluctance of the magnetic circuit changes. It follows that the methods of controlling the two types of machine are quite different, since the control is related to the method of torque production. In general, the control methods used for sinusoidally fed conventional machines are inappropriate for switched reluctance machines.
FIG. 1 shows a typical switched reluctance machine in cross section. In this example, the ferromagnetic stator 10 has six stator poles 12. The ferromagnetic rotor 14 has four rotor poles 16. Each stator pole carries a coil 18. The coils on diametrically opposite poles are connected in series to provide three phase windings. Only one phase winding is shown, for clarity. The control of the switched reluctance machine can be achieved in a variety of ways well known to the person skilled in the art. If information on the angular position of the rotor is available, e.g. from a position transducer, the excitation can be applied as a function of the position. Such machines are often referred to as “rotor position switched machines”.
A typical switched reluctance drive is shown in FIG. 2. In this example, the machine 36 corresponds to that shown in FIG. 1. The three phase windings, A, B and C, are switched in turn onto a DC supply V by a set of power electronic switches 48. The moments (i.e., the rotor positions) at which the switches operate are determined by the controller 38, which may be implemented either in hardware or in the software of a processing device such as a microcontroller or digital signal processor. The control signals are sent to the switches via a data bus 46. Closed loop current feedback is provided by sensing the phase currents using a current sensor 44 and feeding back a signal proportional to phase current which is compared to a demanded current iD. The control algorithms may include a proportional (P), proportional-plus-integral (P+I), time optimal, feedback linearised, proportional/integral/derivative (PID) function, or one of many others as is well understood in the art. It is also common for an outer control loop of position or speed to be provided by feeding back a rotor position signal from a position detector 40.
In operation, a signal corresponding to current demand 42 is provided to the controller. This regulates the current in the windings, according to the particular control scheme adopted, to produce the desired output from the machine.
The performance of a switched reluctance machine depends, in part, on the accurate timing of phase energisation with respect to rotor position. Detection of rotor position is conventionally achieved by using a physical rotor position transducer (RPT) 40, shown schematically in FIG. 2, such as a rotating toothed disk mounted on the machine rotor, which co-operates, for example, with an optical or magnetic sensor mounted on the stator. A pulse train indicative of rotor position relative to the stator is generated and supplied to the processing device, allowing accurate phase energisation. Alternative methods of position detection include the so-called “sensorless” methods, in which there is no physical position transducer and the position is deduced from measurements of one or more other parameters of the machine.
Since current in the windings is relatively easy to measure, closed-loop control of the machine is conventionally accomplished by monitoring and controlling the energising current in the windings. However, the desired output of the machine is usually torque, position or speed, and current has a highly non-linear relationship to all of these. The result is that current control techniques generally incur inaccuracies in the output, such as torque ripple, position error and/or speed error. Many current control schemes have been devised to address these shortcomings, as will be discussed further below.
Many different power converter topologies are known, several of which are discussed in the Stephenson paper cited above. One of the most common configurations is shown for a single phase of a polyphase system in FIG. 3, in which the phase winding 32 of the machine is connected in series with two switching devices 21 and 22 across the busbars 26 and 27. Busbars 26 and 27 are collectively described as the “DC link” of the converter. Energy recovery diodes 23 and 24 are connected to the winding to allow the winding current to flow back to the DC link when the switches 21 and 22 are opened. A capacitor 25, known as the “DC link capacitor”, is connected across the DC link to source or sink any alternating component of the DC link current (i.e. the so-called “ripple current”) which cannot be drawn from, or returned to, the supply. In practical terms, the capacitor 25 may comprise several capacitors connected in series and/or parallel. Where parallel connection is used, some of the elements may be distributed throughout the converter. A polyphase system typically uses several “phase legs” of FIG. 3 connected in parallel to energise the phases of the electrical machine independently.
The phase inductance cycle of a switched reluctance machine is the period of the variation of inductance for the, or each, phase between common points in successive cycles (for example between inductance maxima when the rotor poles and the relevant respective stator poles are fully aligned). As explained in the Stephenson paper cited above, the maximum inductance region is centred around the rotor position where a pair of rotor poles are fully aligned with a pair of stator poles. Similarly, the minimum inductance region is centred around the position where the interpolar axis on the rotor is aligned with the stator pole axis, as shown in FIG. 1.
At low speeds, switched reluctance systems generally operate in a current-controlled or “chopping” mode. A hysteresis current controller using “hard” chopping is often used, as explained in the Stephenson paper referred to above. This is illustrated in FIG. 4(a) where the current cycles between an upper hysteresis level Iu and a lower hysteresis level Il in a conduction region of the phase in question, between the switch-on angle θon at which the phase is energised and the switch-off angle θoff at which energisation is removed. An alternative control regime is “soft” chopping in which only one switch is opened when the current reaches its upper level. The current then decays much more slowly through the winding, the second switch and one diode. This is shown in FIG. 4(b). Depending on the capability of the switches and the current controller, the width of the hysteresis band can be reduced until the current effectively becomes flat. If the angular speed of the rotor is slow, then the angle traversed by the rotor between switch-on and reaching the desired level is very small, so that the current waveshape appears to be rectangular, as will be discussed below. Other types of current controllers are well known in the art, for example those described in EP-A-0769844, which is incorporated herein by reference, off-time controllers, constant frequency controllers, etc., and will not be further described here.
At higher speeds, switched reluctance systems typically operate in the “single-pulse” mode of energisation instead of the chopping mode. This is also explained in the Stephenson paper referred to above.
Thus, systems generally use a chopping mode at low speeds and a single-pulse mode at higher speeds. The upper and lower chopping current levels are normally set to values above the expected peak current of the single pulse mode, so that these parameters do not interfere with single-pulse operation. It is known to set the upper current level to a value which would act as a “safety net” so that if a fault condition developed in the drive, the current would exceed this upper level and cause one or more switching devices to be opened, thereby limiting the current to a safe value.
While motoring operation has been assumed in the above discussions, it is well-known that switched reluctance machines operate equally well in the generating mode, in which the current waveforms are generally mirror images of the motoring waveforms.
Unlike some other types of electrical machine, the switched reluctance machine does not generally have a linear relationship between torque and current. The reasons for this are discussed in greater detail in the Miller book and the Stephenson paper cited above. The relationship is illustrated in FIG. 5, where the so-called static torque for one phase of the three-phase machine of FIG. 1 is shown for a constant current applied over a rotor angle of 45°. For a current low in the working range of the machine (say below 10%), the torque would be almost rectangular. However, as the flux and current levels are raised, the magnetic properties of the iron carrying the flux become significantly non-linear and the shape of the torque becomes rounded. The shape of torque curve shown is typical for the rated current for the machine.
It will be realised by those skilled in the art that the current waveform shown in FIG. 5 is idealised, since the practical waveform would typically have some chopping ripple superimposed on the average current shown.
FIG. 6 shows the relationships between the torque curves of the different phases of the machine. For the machine with 6 stator poles and 4 rotor poles, the angular displacement of the curves (the so-called “ε angle”) is 30°. The simplest method of providing continuous torque from the machine as it rotates is to switch on a phase when the torque curves cross and to switch it off and switch on the next phase after the ε angle has been traversed. This is illustrated in FIG. 7, again for a constant phase current. While this is a simple control regime to implement, it has the obvious drawback of producing a large torque ripple. The minimum torque available at any angle is called the ε torque and is shown in FIG. 7. The average torque produced over the ε angle will be somewhere between the peak torque and ε torque, depending on the exact shape of the curve. This method has another drawback in that each phase is only being used for one third of the phase period, so the utilisation of the stator and the electronic controller is poor.
To overcome these disadvantages, it is known to excite every phase whenever it has the potential to produce torque in the desired direction. For the 3-phase machine shown, this gives an excitation pattern of Phase A alone for 15°, followed by A+B for 15°, followed by Phase B alone for 15°, etc. It is illustrated in FIG. 8 (which neglects any mutual interaction between the phases). This pattern is known variously as phase overlap or as “1½ phases on”. It will be seen that the ε torque has been effectively doubled (because two phases are now producing identical torque at the previous crossover point and the slopes of the torque curves have approximately the same magnitude). Because the peak torque is unchanged, the torque ripple is much smaller and the average torque has significantly increased. Each phase is now used for one half of the phase period, so the stator utilisation is increased.
Although this excitation regime is adopted for many drives, there are some applications where a smoother torque is required without undue penalty on the current rating for the devices used to control the currents.